gfscombine3D: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/bin/gfscombine3D - from package: gerris-mpi
/usr/bin/gfscombine3D - from package: gerris

You may install the required package with command apt-get install {package}
Error in other languages:
gfscombine3D: bevel nie gevind nie
gfscombine3D: comandă negăsită
gfscombine3D: comando não encontrado
gfscombine3D: commande introuvable
gfscombine3D: command not found
gfscombine3D: käsku ei ole
gfscombine3D: không tìm thấy lệnh
gfscombine3D: komanda nerasta
gfscombine3D: Kommando nicht gefunden.
gfscombine3D: kommandot finns inte
gfscombine3D: komut yok
gfscombine3D: nie znaleziono polecenia
gfscombine3D: níor aimsíodh an t-ordú
gfscombine3D: no se encontró la orden
gfscombine3D: no s'ha trobat l'ordre
gfscombine3D: opdracht niet gevonden
gfscombine3D: parancs nem található
gfscombine3D: perintah tidak ditemukan
gfscombine3D: príkaz nenájdený
gfscombine3D: příkaz nenalezen
gfscombine3D: команда не найдена
gfscombine3D: командата не е открита
gfscombine3D: 命令找不到
gfscombine3D: コマンドが見つかりません